Hi,
I brought som NOS hafler DH-220 drivers boards and I'm looking for information on the voltage output of the power transformer. I've got conflicting information from google.com. Some say it's 42V ct. and others say it's 45V ct. Any body knows for sure?
The troubleshooting section in the schematic lists a max DC voltage of 65V DC so that puts it more towards 45V ct catagory. I just wanted to be sure.

As Duo says, I don't think you're going to have a problem either way. As I recall, the DC voltage going to the boards is about +-60V or so. You'll drop a volt or so going through the rectifier, and the bias will draw the voltage down another whisker. I'd say 45-0-45, i.e. 90VCT.
(A 45V center tap transformer would give you 22.5-0-22.5 and yield rails on the order of 30V or so. Not necessarily what you want, unless you intend to rebias for class A...)

Sorry. I meant 90V ct. I have a spare 80V ct laying around. Can I use that to test the driver board? I mean will it cause damage?
Another question is I see that there's a small by-pass cap (C-401) at the gate-source pins of one of the N-Channel Fets (only one), but nothing on the other parallel FET and nothing on the two P-Channel Fets. Question is what's that cap for and how do I know which FET to connect this cap to.

Go for the lower voltage. At worst, you'll lose a couple of watts, pretty insignificant.

I don't have the schematic, but from your description, that cap was used in an attempt to make up for the non-complementary nature of the MOSFETs used in this circuit (Early Hitachis, IIRC). My guess would be that the cap is something like 200pF or so?

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